Syphilis is a sexually transmitted bacterial infection that can be easily cured with antibiotics in the early stages. But many people do not have symptoms early on, or don't recognize the symptoms, and continue to transmit the infection.

In the U.S., syphilis has been on the rise since 2000, when the national rate hit an all-time low of 2.1 cases per 100,000 people.

That increase has been largely among men, who had a rate of just under 8 cases per 100,000 in 2009 (versus 1.4 cases per 100,000 women), according to the CDC. And studies have suggested that gay and bisexual men now account for a majority of new syphilis cases.

Health officials are concerned about the resurgence not only because of syphilis itself, but also because the infection makes people more vulnerable to contracting HIV, the virus that causes AIDS.

Now the new findings, reported in the Annals of Internal Medicine, show that minorities -- and young men, in particular -- are being hit hardest by syphilis.

Using data from 27 states, CDC researchers found that between 2005 and 2008, the syphilis rate among black gay and bisexual men rose at an 8-times faster clip compared with their white counterparts.

Hispanic gay and bisexual men, meanwhile, had more than twice the increase of white men.

By 2008, the syphilis rate among black gay and bisexual men was 19 per 100,000. Those figures were just over 7 per 100,000 among Hispanic men, and 4 per 100,000 among white men.

What's more, the CDC says, there has been a shift in the age group most affected by syphilis.

Ten years ago, outbreaks of the STD were largely reported among gay and bisexual men in their 30s.

But since 2005, teenagers and men in their 20s have shown the biggest increase in syphilis cases.

And 20- to 29-year-olds had the highest rates in 2008 -- at around 12 cases per 100,000.

Racial disparities in syphilis and other STDs have long been seen among U.S. men in general. So the current findings are not especially surprising.

But the magnitude of the racial gap in this study is concerning, said lead researcher Dr. John R. Su, a medical epidemiologist at the CDC.

It's hard to pin down the precise reasons, Su told Reuters Health in an interview.

But he said it could reflect factors like lower incomes and education levels, and poorer access to healthcare -- in other words, some of the same race-related disparities seen among Americans in general.

As for young gay and bisexual men, some recent studies suggest that they are increasingly engaging in risky sex, including having unprotected intercourse and multiple partners.

The bottom line for gay and bisexual men is awareness, according to Su.

"First, you have to know you're at risk," he said. "Then have a frank discussion about it with your healthcare provider."

It's recommended that all sexually active gay and bisexual men be tested at least once a year for syphilis and other STDs, including HIV and gonorrhea, Su said. Staying in a monogamous relationship with a partner whose been tested, and using condoms consistently, reduce the chances of contracting syphilis and other STDs.

But the recent re-emergence of syphilis may require "novel" public-health responses, according to an editorial published with the study.

"For example, many MSM (men who have sex with men) with newly diagnosed syphilis or HIV met their sexual partners recently on the Internet," write Dr. Kenneth H. Mayer and Matthew J. Mimiaga of the Boston-based Fenway Institute, which specializes in healthcare for gay, bisexual and transgender patients.

So education and screening efforts, they say, need to target men where they go: online, and at clubs, bars and bathhouses.

Su agreed that more-innovative steps, like using the Internet or text-messaging to spread health messages, are promising.

He also pointed to programs that use "peer educators" -- lay people who are trained to help get health messages out to their local community.

Along with getting regular screening, at-risk men should know the potential signs of syphilis infection. The first is usually a single painless sore on the genitals, rectum or mouth (wherever the bacterium entered the body). Some weeks after that, a non-itchy rash may develop on different parts of the body. Some people also have flu-like symptoms such as a fever, sore throat, body aches and swollen glands.

About 15 percent of people with untreated syphilis eventually develop long-term complications, according to the CDC. Those include damage to the brain, nerves, heart and blood vessels that can prove fatal.